The present invention relates to an improved magnetic recording medium for displacement detectors, and more particularly relates to an improvement in a magnetic recording medium for displacement detectors such as magnetic rotary encoders or magnetic linear scales which detect the amount of an angular or linear displacement of a mobile object.
In a magnetic rotary encoder, a typical magnetic recording medium, used for industrial devices such as robots, is a magnetic disc which rotates with a mobile object. Such a magnetic disc is generally provided with a ferromagnetic material attached on its surface or made of a ferromagnetic material. Graduations spaced at constant interval and one or more standard marks are magnetically written in the surface of the magnetic disc. One or more magnetic resistor elements are arranged facing the graduations and the standard mark or marks and generate output signals in response to the graduations and the standard mark or marks in order to provide information regarding the rotation angle, i.e. the total amount of rotation, of the mobile object.
There is a type of encoder which includes a plurality of standard marks and necessitates discrimination between different standard marks. The following method is conventionally employed in order to effect such a discrimination.
A magnetic disc of such an encoder generally includes two concentrically formed magnetized tracks. The outer magnetized track includes a number of juxtaposed magnetic graduations whereas the inner magnetized track includes, for example, four magnetic standard marks circumferentially spaced from each other at equal intervals. Each graduation is written in in the form of a relatively long pulse having a width "T" and each standard mark is written in the form of a relatively short pulse having a width "t". The first standard mark is located at the rising position of one graduation, the second standard mark is located at a position 90.degree. out of phase from the first standard mark, the third standard mark is located at a position 180.degree. out of phase and the fourth standard mark is located at a position 270.degree. out of phase from the first standard mark. With this construction, the above-described discrimination of each standard mark can be carried out by detecting the phase-difference between the specified graduation and the standard mark.
In order to successfully practice the above-described discrimination, the following relationship must exist between the pulse widths "T" and "t": EQU 0&lt;t&lt;2T/n (1)
From this relationship, it is clear that an increase in the number "n" of standard marks necessitates a corresponding decrease in pulse width "t" of the standard marks. Such a decrease in pulse width renders the writing-in operation very difficult. In addition, the standard marks and the graduations both have to be read out for the discrimination.